The invention can be used to good advantage in a cigarette packing machine, at the outfeed of which there is a stabilising and drying conveyor comprising two conveyor belts having two parallel, facing branches that move in a given direction and form between them a channel extending along a path of given length and whose width is substantially the same as the width of the cigarette packets.
The packets are held in contact with each other, usually along their large lateral faces, between the two conveyor branches so that they move along the conveyor, which is long enough to enable the glued parts of the packets to dry completely.
At the end of the conveyor, there is usually a printing device, of the laser or inkjet type, designed to print a sequence of letters and numbers forming corresponding codes on visible portions of the packets, following a control signal applied to it by a sensor that detects each single cigarette packets as it passes the printing zone.
These sensors, in order to be able to detect the line dividing two consecutive packets that are in close contact with each other and thus to identify the single packets, must be extremely sensitive and have a low depth of field. This feature, however, makes sensors of this kind unreliable. Indeed, a packet that moves even very slightly out of position may lead to a sensing error. Thus, for example, when a defective packet is rejected, a break is created in the row of packets moving along the feed channel. When this happens, the packets adjacent to the break may be angled slightly relative to the two facing branches of the conveyor, causing the sensor to issue an incorrect signal.
Furthermore, since the packets have to be held closely together between the conveyor belts for the full length of the conveyor so that the glued parts of the packets can dry properly without coming apart, so as to obtain a high quality end product, the parts of the packets that can be printed on are extremely limited.